Genes of any origin carrying any type of information can be introduced into bacterial cells by means of in vitro DNA recombination, as it is widely known. Stable maintenance of the foreign DNA and the expression of the information for which it codes can be carried out by means of appropriate carrier molecules, the expression vectors. When expressing a gene coding for a protein in bacterial cells, the enzymes of the bacterium firstly synthesize an mRNA copy on the DNA template during the course of transcription and a polypeptide chain afterwards, using the information coded in the RNA message, during the course of translation. Transcription is initiated at a DNA region called a promoter. RNA polymerase recognizes this DNA region and binds to it before the initiation of transcription. In vitro DNA recombination methods can be used economically in practice for the production of proteins only if these proteins are produced in relatively high amounts, that is both transcription and translation are effective. Therefore it is very important for promoters to be "strong" (besides other needed features), that is a suitably high level of transcription must be initiated on them.
Different expression vectors have been constructed by the use of many kinds of promoters, e.g. lac, trp, bla, llp, lambda p.sub.L promoters. The promoter of the lac operon has been widely used in practice mainly becouse of its well controlled functioning though it is a relatively week promoter. So called hybrid promoters of different origin have been described e.g. in European Patent Application No. 82302532.5 (publication number 0 06.sup.-- 540). These promoters are characterized by the fact that the so called -35 and -10 regions (which are) the most important regions of a well functioning promoter) are of different origin, that is the two promoter parts of different origin are joined together in the region between the -35 and -10 regions. According to the abovementioned patent application, expression vectors constructed with the use of such hybrid promoters are excellent for industrial purposes. Since their functioning is well controlled and they make efficient transcription possible at the same time.